Scalability CheatSheet— Part 3 — PAXOS
Reading is now difficult you need to read all your journaling, so from time to time you create a snapshot of state, you see so now you have a snapshot and you augment it with your appended only jouranling.
Just Read — When you read you just read, you don’t lock, you don’t care about the world, it’s like you are high you just read and reading now does not disturb any writes just read without disturbing writes, its immutable, all is cool dude, we are reading.
Collision — What happens if two distributed machines try to write at the very same time same timestamp and different content on same key? OMG we just have a problem even in append only journaling, let’s scratch our heads and see what we can do, calling turing might be a good idea, but he didn’t answer any of my calls for the last couple of years, so we’ll have to figure something out.
Eventually Consistent — Less is the new more — A secret weapon we came upon which basically means, we can’t have the best of all world and we don’t need to. For our cases we are going to satisfy with less. As each participant has its own local strongly consistent store, they update each other about.. updates. With eventual consistency you build a map with all nodes and update others when something happens and can change route no one node has precedence over others who has same data. —
The Part Time Parliament — Quorum — Paxos is doing it in manner of reaching an agreement between chosen members, for example you need at least half the nodes to agree on it. The Paxos uses the part time parliament example — in order to make a change you have to get the agreement of the majority of the paxons. Now you cannot have two set of paxons both larger than half the group having the same decision on change .
Master election — Master election is just yet another example of agreeing on something, in this case you agree on master, it’s an expensive, strongly consistent store used to decide who is in charge then if you need something you contact him, but once you all agree on a master it simplifies your processes when you need to read or write you just contact me, so it eliminates further agreements on every read and write.